The invention relates to a semiconductor storage device and a method for manufacturing the same.
In conventional non-volatile semiconductor storage devices, regions between word lines that have a layer structure including a tunnel oxide film, a floating gate electrode, an interpoly insulating film, and a control gate electrode are filled using an oxide film or a nitride film. However, as devices have shrunk to microscopic sizes, various problems have arisen. One problem is that because the interval between the word lines has been reduced, variation among threshold voltages of the floating gate electrodes has increased due to parasitic capacitances generated between the floating gate electrodes of adjacent word lines, and writing speed has dropped due to parasitic capacitances generated between the floating gates and diffusion layer. Another problem is that the material embedded between the electrodes breaks down as a result of the large electric field applied between the electrodes.
One proposed solution for such problems is to provide air gaps (spaces) between the word lines to reduce the parasitic capacitances thereby suppress the variation in the threshold voltages of the floating gate electrodes and the reduction in the writing speed.
A known method for forming air gaps is to use an organic material as a sacrificial film and remove the sacrificial film by ashing (see, for example, Japanese Patent Laid-Open No. 1-137651). However, when such a method is applied to form the air gap between the word lines, there is a problem that the carbon or the like included in the organic material degrades the tunnel oxide film.
A further known method of forming the air gaps is to form a spacer made up of a silicon nitride film to cover the word line, form a sacrificial film made up of a silicon oxide film up to a predetermined height between the word lines, form a mini-spacer made up of a silicon nitride film on the sacrificial film, and remove the sacrificial film while preserving a selection ratio between the sacrificial film and the silicon nitride film (see, for example, Daewoong Kang et al, The Air Spacer Technology for Improving the Cell Distribution in 1 Giga Bit NAND Flash Memory, IEEE NVSMW2006, p 36-p 37).
However, this method has a problem in that hot carriers are generated when a silicon nitride film is used as a spacer. A further problem is that hydrogen included in the silicon nitride film during the heating process causes degradation of the tunnel oxide film.
Thus, conventional methods for forming the air gap have a problem in that the tunnel oxide film is degraded, causing a drop in the reliability of the semiconductor storage device.